The present invention pertains to the field of metal plating and, more specifically, to a method for applying a non-ferrous plated undercoating to a ferrous metal substrate to provide improved corrosion resistance, while facilitating conventional top-plating thereupon, and to the novel non-ferrous plated undercoating provided thereby.
In the art of metal plating, it has been well known and generally recognized that the corrosion resistance of ferrous metal substrates, as well as the adherence of conventional top-plating metals thereto, can be significantly improved by provision of various undercoatings thereupon.
Perhaps the most simple such undercoating known in the art is a single plated layer of a non-ferrous metal, such as zinc, cadmium, nickel or brass, which is selected for use based upon the particular application to be made of the substrate article. In some applications, metals such as zinc or cadmium are utilized to provide "sacrificial plates," in that they protect the metal by themselves being preferentially corroded, instead of the ferrous metal substrate. In other applications, plates of metals such as nickel and brass are utilized as "barrier plates," which form an impervious layer that, for a reasonable time, can prevent penetration of corroding agents in the atmosphere or environment to which the coated substrate is exposed.
As the art developed, various modifications of the single plate type coating developed. For example, one of these modifications involved use of a dual nickel coating in which the lower layer has one type of crystalline structure, while the upper layer comprises nickel of yet another crystal structure. There have also been refinements involving provision of three separate layers of nickel.
Another improvement has been the utilization of a micro-porous layer of nickel and chromium to spread the potential on the surface and prevent pits from forming. The use of multiple plated layers of various metals is also known in the art. For example, during the period of the Korean War in which nickel became scarce, it was found that brass could be utilized as an undercoating substitute for all, or part, of nickel. In this regard, white brass, which is a high zinc content homogeneous alloy of copper and zinc, was found to provide a degree of protection above and beyond that available from pure barrier-type protection. Thus, it was realized that combination of sacrificial layers such as zinc, cadmium or brass with copper, nickel or chromium could be accomplished to provide improved performance.
One of the more recent developments in the field has been the provision of a barrier/sacrificial type plate to be utilized on articles of a ferrous metal, such as fastening devices, for example, nuts and bolts and various sundry parts for motor vehicles. This type of coating generally utilizes a barrier-layer of cadmium, cadmium-tin alloy, or a layer of cadmium and a layer of tin, and is typically deposited over a copper strike on the ferrous metal substrate, followed by a "sacrificial" layer, such as zinc, followed by a second layer of copper, and, finally, a conventional topcoating, such as nickel or chromium. While this type of undercoating provides improved corrosion resistance, it has disadvantages in that the resulting undercoating contains a metal of considerable toxicity, namely cadmium, which is also quite soft and malleable. These disadvantages are the source of difficulties both in manufacture and in use of the plated articles by the consumer.
Thus, in the present state of the art, it is known that brass, a homogeneous alloy of copper and zinc, can be utilized as such, for "barrier plate" type protection against corrosion of an underlying ferrous metal. However, the prior art contemplates only the use of the well-known homogeneous brass alloys, such as red brass, yellow brass, white brass or the like. Thus, while several U.S. patents, namely U.S. Pat. Nos. 2,115,749 (Rubin), 2,392,456 (Brown) and 2,490,700 (Nachtman), disclose use of a layer of brass on a ferrous substrate, they specifically indicate that a homogeneous brass alloy must be utilized. While each of these patents describes a somewhat different method of forming brass on the ferrous substrate by application of adjacent layers of copper and zinc, followed by the application of heat to cause alloying, each clearly discloses formation of a homogeneous alloy of a specific, uniform composition of copper and zinc resulting from complete diffusion (Nachtman) or complete alloying (Rubin and Brown) of the serially plated layers of copper and zinc. Yet the prior art has failed to discover the superior advantages and corrosion resistance provided by the multi-layer undercoating of the present invention comprising a novel combination of sacrificial and barrier type protection utilizing zinc and a novel zone of non-homogeneous copper-zinc alloy.